Hydraulic actuator

ABSTRACT

The invention concerns an hydraulically powered mechanism of the kind having a piston assembly which is arranged to be driven by hydraulic fluid acting on it in a working zone and an item of equipment which is to be driven by the piston assembly. The piston assembly presents a face which acts at an interface against a complemental face on the item of equipment. A fluid passage is provided to supply hydraulic fluid from the working zone to the interface for lubrication purposes. In a specific application of the invention it may be used in the rotary indexing mechanism of an hydraulically powered, typically a water-powered, rockdrill.

BACKGROUND TO THE INVENTION

THIS invention relates to hydraulically powered mechanisms.

The specifications of South African patents 92/4302 (Trade Firm 57 (Pty)Limited) and 96/7417 (White Manufacturing (Pty) Limited), the contentsof both of which are incorporated herein by reference, describehydraulically powered rotary indexing mechanisms for rotationallyindexing the drill steel of a water-powered rockdrill. In each casedifferential hydraulic forces acting on opposed piston assemblies areused to achieve rotary oscillation of a rotor which transmits indexedrotary motion to the drill steel via a one-way clutch and chuck in whichthe drill steel is engaged.

The piston assembly in each case includes a strut or plunger which has aspherically shaped end seating in a complemental socket in a radiallyprojecting lug or ear on the rotor. A problem inherent in each of theseknown mechanisms is the fact that the spherical interface between theplunger or strut and the rotor socket is an area of high wear.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided anhydraulically powered mechanism comprising a piston assembly arranged tobe driven by hydraulic fluid acting on it in a working zone, an item ofequipment which is to be driven by the piston assembly, the pistonassembly presenting a face which acts at an interface against acomplemental face on the item of equipment, and a fluid supply passagefor supplying hydraulic fluid from the working zone to the interface.

The piston assembly may include a piston and a strut which acts betweenthe piston and the item of equipment with a portion of the passageextending through the strut to the interface. The interface may be aspherically curved interface between a spherically curved end of thestrut and a spherically curved socket in the item of equipment, and thespherically curved end of the strut can be formed with a flat. Thepiston itself may be cup-shaped to receive an end of the strut remotefrom the interface, a portion of the passage extending through the baseof the piston, from the working zone, to communicate with that portionof the passage extending through the strut.

There may be piston assemblies acting at spaced apart interfaces on theitem of equipment and a port extending through the item of equipment toprovide hydraulic communication between the interfaces. This wouldtypically be the case where the mechanism forms part of a rotaryindexing mechanism of an hydraulically powered rockdrill which serves torotate a drill steel with indexed rotation. In this case, the item ofequipment will be a rotor with the complemental face provided by aradial lug of the rotor.

In an hydraulically powered rockdrill, another aspect of the inventionprovides a rotary indexing mechanism for rotationally indexing a drillsteel attached to the rockdrill, the mechanism comprising:

a rotor,

opposed piston assemblies arranged to be driven by hydraulic fluidacting on them, the piston assemblies presenting faces which act atrespective interfaces against complemental faces on the rotor thereby todrive the rotor in rotary oscillation.

means to convert rotary oscillation of the rotor into indexed rotationof the drill steel, and

one or more fluid supply passages for supplying hydraulic fluid to theinterfaces.

Typically in this application the piston assemblies are arranged to bedriven by hydraulic fluid acting on them in respective working zones,the mechanism comprising one or more fluid supply passages for supplyinghydraulic fluid to the interfaces from at least one of the workingzones. Typically also, each interface is a spherically curved interfacebetween a spherically curved end of a strut and a spherically curvedsocket in the rotor lug. The spherically curved end of each strut may beformed with a flat and there may be a port extending through the lug toprovide hydraulic communication between the interfaces, the flatsserving to ensure that such communication is maintained even if thestruts rotate somewhat relative to the lug.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 shows a cross-sectional view of a mechanism according to theinvention; and

FIG. 2 shows a detailed view of the spherical interfaces.

The following drawings show a prior art rotary indexing mechanism asdescribed in the specification of South African patent 96/7417:

FIG. 3 shows a longitudinal cross-sectional view of the fronthead of arock drill and illustrates the rotary indexing mechanism;

FIG. 4 shows a cross-section at the line 2—2 in FIG. 3; and

FIG. 5 shows an exploded perspective view of the rotor and chuck of theindexing mechanism.

DETAILED DESCRIPTION OF THE DRAWINGS

The prior art rotary indexing mechanism depicted in FIGS. 3, 4 and 5will firstly be described.

In FIG. 3, the fronthead of a rock drill is indicated by the referencenumeral 100. The fronthead includes a casing 112 and a chuck 114 whichis supported in the casing by bearings 116 which allow the chuck torotate freely relative to the casing. As also illustrated in FIGS. 4 and5, the chuck is formed with a hexagonal central:.aperture 118 which inuse receives the rear end of a hexagonal cross-section drill steel 120.

A rotor structure having a rotor 122 is mounted coaxially about anenlarged portion 124 of the chuck 114. In this embodiment, the rotor isof one-piece construction and has a ring portion 126 carrying anupstanding lug 128 and a wrap spring portion 130. As will beparticularly clear form FIG. 5, the wrap spring portion 130 extendsintegrally and coaxially from the ring portion 126. In practice, thering, lug and wrap spring portions are machined from a single length ofsteel stock.

The inside diameter of the rotor is constant in the ring portion 126 buttapers down, in a direction away from the ring portion, in the wrapspring portion 130. In addition, it will be noted that the axialdimensions of the turns of the wrap spring portion, and the pitch of theturns decrease incrementally with increasing distance from the ringportion.

The enlarged portion 124 of the chuck has a constant diameterthroughout. The internal taper of the wrap spring portion is chosen suchthat the ring portion and the initial turns of the wrap spring portion,such as those marked 132 and which are closest to the ring portion, makea small clearance on the chuck while the later turns of the wrap springportion, such as those marked 134 and which are furthest from the ringportion, are slightly smaller in diameter, when relaxed, than the chuck.With this arrangement, there is slight outward deformation of the turns134 when the rotor is mounted on the chuck, so that the turns 134already grip the chuck surface to some extent.

The taper described above is extremely small and is not visible in theview of FIG. 3.

The lug 128 is formed with opposing, part-spherical sockets 136. Thecasing 112 is formed with cylinders 138 and 140 in which respectivehollow pistons 142 and 144 are reciprocable. The hollows in the pistonshave part-spherical bases 143 and accommodate struts 146 and 147 each ofwhich has part-spherical outer and inner ends 148 and 150 respectively.The ends 148 seat complementally in the bases 143 of the hollows of thepistons and the ends 150 seat complementally in the sockets 136.

Ports 152 and 154 lead into the cylinder 138 and 140 respectively. Thepiston 142 is sealed with respect to the cylinder 138 by seals 156 and158. A hole 157 is formed in the casing 112 to allow any hydraulic fluidleaking past the seal 158 to exhaust to atmosphere.

The piston 144 is sealed with respect to the cylinder 140 by a seal 160and carries a sliding bearing 162. Angled holes 145 are formed in thepiston 144 as illustrated.

The pistons 142 and 144 have stepped outer surfaces as will be apparentfrom FIG. 4. In addition, the pistons have external flanges 164 and 166which make a small clearance relative to sections 168 and 170 of thecylinders 138 and 140. The geometry is such that the piston 144-presentsa larger cross-sectional area to the fluid in the cylinder 140 than thepiston 142 presents to fluid in the cylinder 138.

The port 152 is connected to a source of hydraulic fluid, typically minegrade water, at steady pressure, while the port 154 is connected to asource of similar hydraulic fluid at fluctuating pressure. In practice,the port 152 is connected to the source which supplies pressurised waterfor the percussive action of the rock drill while the port 154 isconnected to the working chamber of the rock drill percussion system,and so receives water alternatively at supply pressure and a low exhaustpressure.

The operation of the mechanism illustrated in FIGS. 3, 4 and 5 of thedrawings will now be described.

The smaller piston 142 is supplied continuously with high pressure fluidand thus is subjected throughout to a hydraulically generated forcewhich urges it to the right in FIG. 4. When the cylinder 140 is suppliedwith high pressure fluid the other, larger piston 144 is subjected to ahydraulically generated force which urges it to the left in FIG. 4.

The force acting on the piston 144 is larger in these circumstances thanthe force acting on the piston 142. The hydraulic forces on the pistonsare transmitted to the lug 128 by the struts 146 and 147, with theresultant rotational force driving the rotor 122 in anticlockwisedirection as seen in FIG. 4.

When the controlling rock drill cycle switches over the cylinder 140 issupplied with hydraulic fluid at a low, exhaust pressure. There istherefore a substantial reduction in hydraulic pressure acting on thepiston 144, and hence a substantial reduction in the force which urgesthis piston to the left in FIG. 4. The piston 142 is however stillsupplied with the same pressure and is accordingly urged to the rightwith the same force as before. In this situation, the force acting onthe piston 142 is greater than the force acting on the piston 144 andthe resultant force drives the rotor 122 in a clockwise direction inFIG. 4.

Therefore as the rock drill hydraulic system cycles the rotor 122 isoscillated alternately in a clockwise and anticlockwise direction. Whenthe rotor rotates in the anticlockwise direction, as viewed in FIG. 4,the turns or coils of the wrap spring portion 130 are “wound up” withthe result that they bind onto and grip the chuck 114 in a strongfrictional engagement. Thus oh this cycle, rotary motion is transmittedto the chuck, causing the chuck and the drill steel to rotate. Theinitial binding of the wrap spring portion onto the chuck is enhanced bythe existing engagement resulting from the internal taper of the wrapspring portion, as described above.

When the rotor 122 is rotated in the clockwise direction the coils orturns of the wrap spring portion tend to “unwind” with the result thatthe wrap spring portion releases the chuck, so that no rotary motion istransmitted from the rotor to the chuck and drill steel. Thus as therotor oscillates in accordance with the controlling rock drill cycle,the chuck is rotated incrementally, i.e. indexed in an anticlockwisedirection as viewed in FIG. 4.

The alternate gripping and releasing of the chuck is aided by theinertia of the spiral wrap spring portion. As the rotor accelerates inthe anticlockwise direction the inertia of the coils or turns of thewrap spring also tend to “wind up” the spiral, whereas on the oppositestroke, the inertial effects tend to “unwind” the spiral.

The movement of the piston 142 and 144 is generally tangential to thechuck, with the rotor driving forces transmitted by the struts 146 and147. The spherical seats at the ends of the struts allow thesecomponents to assume the correct orientations throughout their movementto transmit the necessary driving forces despite the fact that thepistons 140 and 142 undergo straight line motion in their respectivecylinders. Thus an advantage of the use of spherical-ended struts asillustrated is the fact that wear of the chuck bearings 116 can beaccommodated irrespective of direction.

The exact position at which the indexing and reset strokes begin and endis not fixed relative to any given point on the drill casing. This isbecause the rotor oscillates as the drill cycle switches and not at anyfixed point in its travel. At the same time, the load resisting thedriving torque varies greatly from a condition in which the drill steelis jammed in a rock mass which is being drilled to a condition in whichthe chuck is indexed freely with no drill steel in position. As theexternal loads on the indexing mechanism vary the reciprocating pistons140 and 142 may migrate towards one or other end of their associatedcylinders, with the danger that they may impact against those ends.

In order to reduce piston speed and the severity of impacts between thepistons and the ends of the cylinders, hydraulic dashpots are providedby the limited clearance between the flanges 164 and 166 and the section168 and 170 of the cylinders. The expulsion of hydraulic fluid throughthe small annular gaps between the flanges and the cylinders set up aback-pressure which acts on the flanges and retards the motion of thepistons, thereby limiting their maximum speed. Thus although the pistonsmay still impact on the ends of the cylinders, this is at a reducedspeed less likely to cause serious damage.

Although in the embodiment described above, one piston is supplied withhydraulic fluid at constant supply pressure and the other with hydraulicfluid alternately at supply and exhaust pressure, it would also bepossible for both pistons to be supplied alternately with fluid atsupply pressure and at exhaust pressure. The differential piston areaswould still ensure that the desired oscillatory motion of the rotor isobtained.

It should also be noted that this mechanism is not restricted to the useof two opposed pistons as described above. There could, for instance, betwo pairs of pistons or a single piston only working against a sprungplunger or as a double-acting piston. In the latter case, the pistonwould of course have to act in tension as well as compression and thiswould require the ends of the strut to be seated in an appropriatemanner to apply both tension and compression.

In the mechanism described above, the ring portion and wrap springportion are formed in one piece. In other mechanisms it is possible forthe wrap spring portion to be formed as a separate component which isattached to the ring portion so as to rotate with the ring portion. Thiscould, for instance, be achieved with a wrap spring portion having itsend located inside the ring portion and made rotationally fast with thering portion by means of a transverse pin or like which extends from onecomponent and lodges in an opening in the other component.

The present invention will now be described with reference to FIG. 1 andFIG. 2 of the drawings.

FIG. 1 shows a rotary indexing mechanism 10 of the kind described in thespecification of South African patent 96/7417. The rotary indexingfunction of the illustrated mechanism 10 is exactly the same asdescribed in this patent specification reproduced above.

The piston assemblies of the mechanism 10 are indicated by the numerals12 and 14 respectively. The piston assembly 12, which can be referred toas the index piston assembly, includes a hollow, cup-shaped piston 16 ofrelatively large cross-sectional area in a cylinder 18 supplied withfluctuating hydraulic pressure through a port 20 and a strut 22 havingfront and rear spherical ends 24 and 26 respectively. The rear sphericalend 26 seats complementally in the piston 16 while the front sphericalend 24 seats in a complementally spherical socket 28 in a radial lug 30on a rotor 32 which, via a non-illustrated clutch, acts on a drill steelchuck 34. The piston assembly 14, which can be referred to as the resetpiston assembly, is similarly configured with a piston 38 of relativelysmall cross sectional area and strut 40 with front and rear sphericalends 42 and 44. The front spherical end 42 seats complementally in aspherical socket 46 in the opposite side of the lug 30.

Referring to FIG. 2, the spherical interfaces between the frontspherical ends 24 and 42 and the associated sockets 28 and 46 aredesignated with the numerals 48 and 50 respectively.

A feature of the mechanism 10 which is not described in thespecification of patent 96/7417, and which is the subject matter of thepresent invention, is a passage 52 which extends through the pistonassembly 12 to place the port 20 in hydraulic communication with thespherical interface 48. It will be noted that the passage extendsthrough ports 53 in the base of the piston 16 and that the remainder ofthe passage extends axially through the strut 22 to the front sphericalface at the end 24.

In operation of the mechanism 10 pressurised hydraulic fluid supplied tothe cylinder 18 through the port 20 can flow through the passage 52 tothe interface 48. At the interface, the hydraulic fluid, i.e. water inthe case of a water-powered rockdrill, provides a wear-reducing wettingand lubricating action between the mating spherical surfaces due toweeping and slight leakage as the mechanism oscillates.

Referring again to FIG. 2, it will be seen that the spherical surface atthe front end 24 of the strut 22 is formed with a flat 54. The area ofthe flat is selected to ensure that as the front end 24 rotates slightlyrelative to the socket 28 during reciprocation of the piston 16 andstrut 22, there is always communication between the passage 52 and theinterface 48. Although not visible in FIG. 1 a similar flat is providedon the spherical surface at the rear end 26 of the strut 22 to ensurethat there is always hydraulic communication between the respectiveportions of the passage 52 which extend through the strut 22 and thepiston 16.

A passage corresponding to the passage 52 could be provided in thepiston assembly 14 in order to supply pressurised hydraulic fluid to thespherical interface 50. This would require fluid communication to beprovided between the supply port 58 and the interior of the cylinderbehind the piston 38.

In view of this and furthermore in view of the difficulty and cost offorming a small diameter bore longitudinally in the strut, it ispreferred merely to provide a port 56 which extends through the lug 30to place the interfaces 48 and 50 in hydraulic communication with oneanother and hence to provide the desired wetting and lubricating actionat the interface 50.

From FIG. 2 it will be seen that the spherical surface at the front end42 of the strut 40 has a flat 60 similar to, and for the same purposeas, the flat 54. A similar flat is provided at the rear end of the strut40 as well, for the purpose described above.

In practice, provided that the areas of the various flats is smallerthan the working areas of the associated pistons, the pressurisedhydraulic fluid will not be able to urge the pistons, struts and socketsapart from one another.

It will be understood that similar provision for wetting and lubricationof the spherical interfaces could also be made in the case of a rotaryindexing mechanism of the kind described in the specification of SouthAfrican patent 93/4302. In fact, in this case, the formation of theinterface supply 52 would be even simpler in that, in the absence of thecup-shaped pistons, there would be a need for a single bore only throughthe plunger.

The invention envisages that the supply of pressurised hydraulic fluidto the interfaces 48 and 50 could also provide a means for hydraulicallypowering a clutch for the rotary indexing mechanism. There could, forinstance, be further porting in the rotor 32 to lead hydraulic fluidfrom the interfaces 48 and 50 to a suitable hydraulic clutch (notillustrated) to ensure that rotary motion is transmitted from theoscillating rotor to the drill steel chuck in one direction only.

In the case of the illustrated embodiment, it will be understood thatthe supply of hydraulic fluid to the interface between the rear ends ofthe struts and the piston sockets will also serve a wetting, lubricatingand hence wear-reducing function.

It will also be understood that as a result of the presence of the flats54, 60 in the relevant faces, those faces are not perfectly spherical.The term “spherical” is nevertheless used herein for convenience.

Although the invention has been described with specific reference to therotary indexing mechanism of a rockdrill it will be understood that theprinciples involved are equally applicable to other hydraulicallypowered mechanisms in which hydraulic lubrication is desirable atworking interfaces where piston assemblies act against items ofequipment which are to be driven by the piston asemblies.

What is claimed is:
 1. A hydraulically powered mechanism comprising: anitem of equipment; a piston assembly arranged to be driven by hydraulicfluid acting on the piston assembly in a working zone, the pistonassembly presenting a face which acts at an interface against acomplemental face on the item of equipment thereby to drive the item ofequipment, the piston assembly including a piston and a strut, thepiston being cup-shaped to receive an end of the strut remote from theinterface and the strut acting between the piston and the item ofequipment; and a fluid supply passage for supplying hydraulic fluid fromthe working zone to the interface, a portion of the passage extendingthrough the strut to the interface and another portion of the passageextending from the working zone, through the base of the piston, tocommunicate with the portion of the passage extending through the strut.2. A hydraulically powered mechanism according to claim 1 wherein theinterface is a spherically curved interface between a spherically curvedend of the strut and a spherically curved socket in the item ofequipment.
 3. A hydraulically powered mechanism according to claim 2wherein the spherically curved end of the strut is formed with a flat.4. A hydraulically powered mechanism comprising: an item of equipment;piston assemblies arranged to be driven by fluid acting on the pistonassemblies in working zones, the piston assemblies presenting faceswhich act at spaced apart interfaces against complemental faces on theitem of equipment thereby to drive the item of equipment; a fluid supplypassage for supplying hydraulic fluid from a working zone to aninterface; and a port extending through the item of equipment to providehydraulic communication between the interfaces.
 5. A hydraulic mechanismaccording to claim 4 wherein the hydraulic mechanism forms part of arotary indexing mechanism of an hydraulically powered rockdrill whichserves to rotate a drill steel with indexed rotation.
 6. A hydraulicmechanism according to claim 5 wherein the item of equipment is a rotorand the complemental faces are provided by a radial lug of the rotor. 7.In a hydraulically powered rockdrill, a rotary indexing mechanism forrotationally indexing a drill steel attached to the rockdrill, themechanism comprising: a rotor, opposed piston assemblies arranged to bedriven by hydraulic fluid acting on the piston assemblies in respectiveworking zones, the piston assemblies presenting faces which act atrespective interfaces against complemental faces on the rotor thereby todrive the rotor in rotary oscillation, means to convert rotaryoscillation of the rotor into indexed rotation of the drill steel, andone or more fluid supply passages for supplying hydraulic fluid to theinterfaces from at least one of the working zones.
 8. A rotary indexingmechanism according to claim 7 wherein each piston assembly includes apiston and a strut which acts between the piston and a lug on the rotorand wherein a portion of a fluid supply passage extends through a strutto an associated interface.
 9. A rotary indexing mechanism according toclaim 8 wherein each interface is a spherically curved interface betweena spherically curved end of a strut and a spherically curved socket inthe lug.
 10. A rotary indexing mechanism according to claim 9 whereinthe spherically curved end of each strut is formed with a flat.
 11. Arotary indexing mechanism according to claim 7 comprising a portextending through the lug to provide hydraulic communication.